Owing to their excellent mechanical and electrical properties as well as other physical and chemical properties and good processability, crystalline thermoplastic polyester resins (e.g., polyalkylene terephthalate resins) have been used extensively as engineering plastics in the production of automotive parts, electric or electronic device parts, and others. Although these crystalline thermoplastic polyester resins can be shaped into a variety of articles singly, depending on the field of use, various reinforcing agents or additives are added thereto to improve their properties, particularly mechanical properties.
However, there is a tendency in such fields toward ever-better properties or characteristics. For example, further improvement in sliding characteristics or dimensional accuracy has been desired. For instance, driving mechanisms for video tape recorders, tape recorders, and printers capable of providing further improved sound or picture quality have been desired. In these operation mechanisms, the dimensional accuracy of the sliding members (e.g., gears) as main driving members is one of the important subjects of further investigation, for their dimensional accuracy is directly and deeply related to the quality of sound reproduction or printing. Further, their initial dimensional accuracy is desired to be maintained even through a long-term service. Materials that show excellent abrasion/wear resistance against gears and shafts (e.g., metal shafts) also have been desired.
From the viewpoint of safety, members constituting the operation mechanisms of printers and facsimiles also are required to be flame-retardant. Until now, resin materials having flame retardancy, sliding characteristics, and molding accuracy all at high levels have no yet been found, and generally, grease is put on a conventional inflammable material or an flammable material such as polyacetal is used. However, a toner for printing is caught by grease and clings to a gear, hindering the driving stability of the gear and remarkably accelerating the wear of the gear. In addition, so that the material of the gear itself is poor in wear resistance against a gear of the same material and in sliding characteristics against metal, if the gear ran out of grease, the performance of the gear will be lowered largely.
Conventionally, a sliding member (e.g., a gear) is made from a material blended with a fluorine-containing resin. However, its reduced gear accuracy due to the anisotropy of the fluorine-containing resin or poor accuracy as a result of mold deposit due to the separation of the fluorine-containing resin from the polyester resin, and the wear of the gear against a member of metal or of the same material have not yet been improved to satisfactory levels, and therefore, materials with improved wear resistance have been desired.
Japanese Patent Application Laid-Open No. 9369/1993 (JP-5-9369A) discloses a resin composition comprised of a polyester-series resin and a polyolefinic resin dispersed therein in a specific state thereby to improve the sliding characteristics and molding processability. However, the combination of a polyester-series resin and a polyolefinic resin does not improve the dimensional accuracy and sliding characteristics largely. Japanese Patent Application Laid-Open No. 35050/1991 (JP-3-35050A) discloses a resin composition comprising, in order to improve the moldability and inhibit the occurrence of surface delamination phenomenon, a thermoplastic polyester resin; an ethylene-.alpha.-olefin co-polymer grafted with an unsaturated carboxylic acid, or an ethylene-unsaturated carboxylic acid ester copolymer; and a bisoxazoline compound. This literature says that the resin composition may comprise a fatty acid ester of a polyhydric alcohol. However, it cannot be said that such resin composition and a shaped article made therefrom always have improved moldability or sliding characteristics. Japanese Patent Application Laid-Open No.150022/1995 (JP-7-150022A) discloses a polyester resin having good sliding characteristics, the resin comprising a crystalline thermoplastic polyester resin, an olefinic polymer comprised of an olefinic polymer fragment and a vinyl-series polymer fragment, and a fatty acid ester obtained from a fatty acid having 12 or more carbon atoms. However, the moldability and sliding characteristics of these resin compositions are still unsatisfactory.
Thus, an object of the present invention is to provide a polyester resin composition with largely improved dimensional accuracy and sliding characteristics, a process for producing the same, a shaped article formed from the same, a resin composition with improved sliding characteristics, and a process for improving sliding characteristics.
Another object of the present invention is to provide a polyester resin composition having good flame retardancy as well as good dimensional accuracy and sliding characteristics, and a shaped article formed therefrom.